Flow rate detection plays an important role in various fields from biochemical detection to national defense security protection. In particular, microfluidics technology has been widely used in cell and biomolecular detection, drug screening, and chemical synthesis and analysis. Currently, various flow rate sensors have been developed, such as electromagnetic flow rate meters, vortex flow rate meters, ultrasonic flow rate meters, and fiber optic flow rate meters. Most of these flow rate meters have problems such as complex detection, susceptibility to interference, and limited flow resolution. Whispering gallery mode (WGM) optical microcavity has the characteristics of high Q value, small mode volume, and high optical field density, and it has attracted much attention in the field of high-sensitivity optical sensing. When combined with microfluidic technology, hollow optical microcavity can be used as a microfluidics channel, high Q value, and strong light matter interaction, so it has extensive research value in high-precision fluid detection and other biochemical sensors. However, there is still room for improvement in flow rate sensitivity. Moreover, higher-order radial modes require high coupling conditions and relatively low excitation efficiency. We propose a flow rate sensor based on graded hollow-core microcavity (GHM) with axial gradient, which achieves direct detection of fluids under micro-pressure conditions using the resonant light field of the microcavity. The sensor has excellent flow rate sensing performance and high application value in high-sensitivity fluid detection, water quality detection, and other fields.

The distribution of fluid rate, pressure, and light field in GHM is analyzed theoretically by using the computational fluid dynamics (CFD) algorithm and finite element analysis of algorithms. High Q value GHM is fabricated. GHM and tapered fiber are coupled precisely and packaged, WGM resonance spectrum is excited and varied according to the change of the flow rate in real time. The flow rate of the liquid is controlled by adjusting the parameters of the peristaltic pump (TJ-3A, the minimum flow rate is 7 μL/min) (flow rates are 15 μL/min, 30 μL/min, 45 μL/min, 60 μL/min, and 75 μL/min, respectively).

Experimental results show that the resonance wavelength shifts towards a longer wavelength as the flow rate is increased (Fig. 5). This is because the sensitivity of the sensor is mainly related to the wavelength of the WGM spectra, the refractive index of the microcavity, and the energy distribution of the WGM in the liquid core region. As the resonance wavelength increases, the flow rate sensitivity will be enhanced. In addition, flow rate sensing is performed on GHMs with different coupling positions and outer diameters (Figs. 6 and 7). The results show that the flow rate sensitivity increases for the larger outer diameter of the GHM. By using an axial gradient structure to increase the interaction between the microcavity WGM light field and the flow field, high-performance flow rate sensing with a flow rate sensitivity of 0.270 pm/(μL/min) (Fig. 7) and a resolution of 1.43 μL/min is obtained (Fig. 8). In addition, the sensor has great stability (Fig. 8) and fast response ability (Fig. 9).

We propose and demonstrate a highly sensitive flow rate sensor based on high-quality graded hollow-core microcavity (GHM), which achieves direct detection of flow rate by the resonant light field of the microcavity's WGM oscillation with minimum intracavity pressure. Firstly, the flow rate and light field of the GHM are theoretically analyzed by fluid dynamics and finite element method. Secondly, GHM with a high Q value (Q>10⁷) is prepared by fused biconical tapering and gas pressure control methods. High Q WGM spectra are excited by high-precision and low-loss coupling between the microcavity and tapered fibers, with five dimensional high-precision displacement stages involved. The flow rate sensing characteristics of the proposed sensor are experimentally investigated by WGM spectra and measured with different cavity sizes and coupling conditions. The maximum flow rate sensitivity is measured to be 0.27 pm/(μL/min), with a flow rate resolution of 1.43 μL/min. This flow rate fiber sensor has high repeatability and real-time performance, and it has potential applications in various fields such as high-sensitivity fluid detection and water quality detection.